It is known that in the course of a flight, violent turbulence related to vertical wind gusts, can cause at the aircraft level:                significant variations in vertical acceleration which, when they are negative, may injure the people in the aircraft;        a significant swerve of the aircraft in terms of altitude, thereby increasing the risk of collision with another aircraft;        spikes in the lift on the airfoil, which test the load limit of the latter; and        discomfort in the cabin.        
Two types of atmospheric turbulence are generally the cause of the disruption experienced by an aircraft, namely:                clear sky turbulence, of CAT (“Clear Air Turbulence”) type, which represents wind shears which often occur without any visible manifestation, generally above 15 000 feet (about 4500 meters). This clear sky turbulence which is of non-convective type, usually appears near the tropopause, especially above mountains and especially in winter. Such turbulence presents a natural risk in flight and can give rise to injuries to the passengers and flying personnel under certain particularly severe conditions. Moreover, such turbulence gives rise to additional fuel consumption (avoidance of the turbulence zone) and contributes to the fatigue of the aircraft and pilots; and        convective turbulence which is often visible through the presence of clouds. Severe convective turbulence appears in fairly humid regions, so that a radar can generally detect it. So, most of the time, this phenomenon can be avoided by aircraft. However, it may happen that pilots are surprised or that they do not have time to evade a zone of convective turbulence. The severest turbulence appears in storm clouds, with ascending and descending currents that may reach several tens of meters per second. This convective turbulence is very localized and less extensive than clear sky turbulence.        
Often, the above phenomena which are localized surprise both the crew of the aircraft and the embedded piloting systems, which do not have time to perform the appropriate maneuvers to attenuate the effects thereof which result therefrom at the aircraft level.
Patent applications FR-2 891 802 and WO-2007/042652 disclose a method and a device for attenuating on an aircraft the effects of at least one vertical turbulence encountered by this aircraft in the course of a flight.
This known method envisages implementing the following series of successive steps:    a) estimating a vertical component of the wind existing outside the aircraft at a current position of said aircraft;    b) with the aid of said vertical component of the wind, calculating:            a first control order for at least one first controllable movable member, in particular a pair of spoilers, which is able to act on the lift of the aircraft, said first control order being such that it makes it possible to minimize the amplitude of the load factors engendered on the aircraft by the vertical turbulence; and        a second control order for at least one second controllable movable member, preferably an elevator, which is able to act on the pitch of the aircraft, said second control order being intended to compensate for the pitch moment engendered by the control of said first movable member;            c) verifying, with the aid of activation logic, whether activation conditions evidencing severe turbulence are fulfilled; and    d) when said activation conditions are fulfilled, transmitting:            said first control order to at least one actuator of said first controllable movable member; and        said second control order to at least one actuator of said second controllable movable member.        
Consequently, these patent applications FR-2 891 802 and WO-2007/042652 envisage acting on the lift of the aircraft, by way of said first movable member, so as to minimize the amplitude of the vertical load factor on the cabin of the aircraft. As regards the second movable member, it makes it possible to compensate for the pitch moment engendered by the control of this first movable member.
These documents FR-2 891 802 and WO-2007/042652 therefore disclose an embedded system for evaluating the level of severity of a vertical turbulence encountered by an aircraft and the deployment of logic using this severity level to automatically control a symmetric deflection of surfaces for directly controlling the lift (first movable members), as well as a deflection of the elevator (second movable member) to compensate for the effects of the pitch moment induced by these surfaces for directly controlling the lift.
The aforesaid known method which thus makes it possible to combat the abrupt variations in vertical load factor, engendered by the violent vertical turbulence of the aforesaid type, presents a few drawbacks, however. In particular:                the envisaged activation logic is based on calculating a probability of occurrence which may be difficult to implement on an aircraft;        the control of the lift, when it is carried out with the aid of a spoiler, is not symmetric, since the spoiler cannot be displaced about a middle position; and        the calculation of the estimation of the wind proposed is not optimal.        
Above all, this routine solution envisages using solely surfaces for directly controlling the lift (said first movable member or members) on the airfoil to reduce the vertical load factor at the aircraft cabin level. The load factor authority of this routine solution is therefore limited in effectiveness. The ensuing attenuation of the longitudinal effects, caused by vertical turbulence, is therefore not optimal.